1. Field of the Invention
The present invention relates to a foam-molded article obtained by sandwiching between molds a parison with a foam layer formed by a process in which an expandable molten resin composition produced by kneading a polyethylene resin melt and a physical foaming agent inside an extruder is extruded and foamed from a die in an area of low pressure, and to a manufacturing method thereof.
2. Description of the Related Art
An expandable molten resin composition is obtained by means of a process in which a thermoplastic resin is heated in an extruder and kneaded to obtain a melt, the melt and a foaming agent are kneaded, and the kneaded material is then cooled to a temperature suitable for foaming. Foam-molded articles produced by processes in which a parison obtained by extruding the resulting composition from a die to an area of low pressure and allowing the composition to foam is sandwiched and molded between molds can be used in ducts, automobile components, containers, electrical appliance enclosure materials, and other products that require thermal-insulating characteristics, soundproofing characteristics, and other characteristics. Methods for manufacturing this type of foam-molded article are cited in Japanese Patent Application Laid-open Nos. H6-312449, 2001-179805, 2001-18283, and other publications.
Conventionally, many polypropylenes and polystyrenes have been studied as raw material resins for foam-molded articles, but the foam-molded articles in which these materials constitute the base resin have insufficient low-temperature impact resistance. In view of the above, polyethylene that is suitable for use at low temperature has come to be studied, as cited in the above-mentioned prior art literature. It is cited in an example given in Japanese Patent Application Laid-open No. H6-312449 that a multilayer parison having a skin layer of high-density polyethylene and a foam layer obtained by foaming a high-density polyethylene with the aid of a chemical foaming agent is molded between molds. In Japanese Patent Application Laid-open No. 2001-179805, a method is cited whereby a multilayer parison provided with a foam layer in which a low-density polyethylene is foamed with the aid of a physical foaming agent, and a polyethylene resin layer on at least the external surface of the foam layer, is molded in a mold so that at least some portions of the innermost pairs of surfaces of the parison fuse with each other to form a foam-molded article having a polyethylene resin layer on the surface of the low-density polyethylene foam layer; and it is also cited that a physical foaming agent is preferably used to obtain a foam layer with a low apparent density. Japanese Patent Application Laid-open No. 2001-18283 cites a method whereby in order to form a foam layer of low apparent density with the aid of a physical foaming agent when sandwiching and molding a parison with a polyethylene foam layer in a mold to obtain a foam-molded article, air is blown into the parison in a state in which the pressure between the internal surface of the mold and the external surface of the parison is kept at a specific level, the interior of the parison is kept at a pressure that is higher than the pressure between the internal surface of the mold and the external surface of the parison, the pressure within the parison is subsequently increased to a specific pressure range to mold the parison, the parison is then cooled while the pressure inside the parison and outside the parison is kept within a specific range, and a foam-molded article is obtained. In Japanese Patent Application Laid-open No. 2001-18283, a specific example is cited whereby a foam-molded article is obtained by molding a parison produced by causing low-density polyethylene to foam with butane, which serves as the foaming agent.
As cited in the prior art literature above, it is known that a foam-molded article may be obtained by molding between molds a parison having a polyethylene foam layer, and it is also known, as cited in Japanese Patent Application Laid-open Nos. 2001-179805 and 2001-18283, that a molded article having a foam layer with a low apparent density may be obtained by using a physical foaming agent.
In view of the above, the present inventors discovered that among polyethylenes, high-density polyethylene is preferably used as the resin that comprises the foam layer in order to impart greater mechanical strength and heat resistance to a foam-molded article. When a chemical foaming agent such as that cited in Japanese Patent Application Laid-open No. H6-312449 is used, a parison having a high-density polyethylene foam layer can easily be obtained, but the apparent density of the foam layer cannot be brought not more than 0.4 g/cm3. Also, in a parison obtained by causing high-density polyethylene to foam with a physical foaming agent as cited in Japanese Patent Application Laid-open Nos. 2001-179805 and 2001-18283, the foam layer has an apparent density of not more than 0.3 g/cm3, and an adequate parison cannot be obtained.